1. Field of the Invention
This invention relates to a catalyst composition and its use. Specifically, it relates to a catalyst composition comprising a crystalline aluminosilicate zeolite and specific metals, and its use in the cracking of non-aromatic hydrocarbons and to the isomerization of C.sub.8 -aromatic hydrocarbons.
More specifically, the present invention relates to an industrially advantageous catalyst composition which, when used in a process for isomerizing C.sub.8 -aromatic hydrocarbons which comprises isomerizing a hydrocarbon feed stock containing a C.sub.8 -aromatic hydrocarbon mixture which has not reached its thermodynamic equilibrium composition and small amounts of non-aromatic hydrocarbons, separating a specific xylene isomer, preferably p-xylene, from the resulting isomerization reaction mixture and recycling the remaining hydrocarbon mixture to the isomerization reaction, causes the C.sub.8 -aromatic hydrocarbon mixture in the hydrocarbon feed stock to reach the thermodynamic equilibrium composition, makes it possible to convert the non-aromatic hydrocarbons, which build up in the process and reduce the efficiency of the isomerization reaction, into components that can be easily removed out of the process, ensures a very little loss of the C.sub.8 -aromatic hydrocarbons, particularly xylenes, and decreases very little in activity after continuous operation over a long period of time, the invention also pertains to its uses.
2. Description of the Prior Art
There has been an increasing demand for xylene, particularly p-xylene, in proportion to the increase of the demand for polyester fibers and films. A typical process for producing p-xylene comprises a step of separating p-xylene from a C.sub.8 -aromatic hydrocarbon mixture by a crystallization method or an adsorption method, a step of bringing the remaining hydrocarbon mixture into contact with a catalyst for isomerizing m-xylene and/or o-xylene to p-xylene to convert the xylenes in the remaining hydrocarbon mixture to a xylene isomeric mixture having a composition close to the thermodynamic equilibrium composition, and a step of recycling the isomeric mixture to the p-xylene separating step.
In the above process for producing p-xylene, it is required to bring the composition of the xylene isomeric mixture in the isomerization reaction mixture to the thermodynamic equilibrium composition as close as possible, inhibit side reactions which cause a loss of xylenes, such as disproportionation reaction and hydrogenating cracking reaction, and to convert ethylbenzene, which because of its boiling point close to the boiling point of the xylenes, is difficult to separate by an ordinary distillation operation, into a higher or lower boiling component easily separable by distillation. Industrially, it is very important to satisfy these requirements in order to increase the efficiency of the isomerization reaction and reduce the cost of the p-xylene production process.
On the other hand, the C.sub.8 aromatic hydrocarbon mixtures heretofore used as materials for isomerization of xylenes are industrially produced by solvent extraction of catalytically reformed oils and thermally cracked in accordance with, for example, the sulfolane method, UDEX method or arosolvan method, and distilling the separated extract. The composition of the C.sub.8 aromatic hydrocarbon mixtures obtained by this method typically consists of 5 to 20% by weight of ethylbenzene, 15 to 25% by weight of p-xylene, 30 to 60% by weight of m-xylene and 15 to 25% by weight of o-xylene.
However, since the above method of producing the C.sub.8 aromatic hydrocarbon mixtures includes the solvent extraction step, the cost of the resulting hydrocarbon mixtures becomes high because extra equipment and energy are required.
In recent years, various attempts have been made to increase the yield of aromatic hydrocarbons such as benzene, toluene and xylene in the reforming of petroleum naphtha. In particular, as a result of improving a catalyst that induces dehydrogenating cyclization of paraffinic hydrocarbons, it became possible to carry out the dehydrogenating cylization under mild conditions at lower pressures. This resulted in an aromatic hydrocarbon mixture with a small content of non-aromatic hydrocarbons.
With this technical background, a method was suggested by which a C.sub.8 -aromatic hydrocarbon mixture having such a relatively small amount of non-aromatic hydrocarons as to make it usable as a starting material for xylene isomerization is obtained from a naphtha reformed oil by distillation treatment alone without using the solvent extraction step (Japanese Patent Publication No. 47231/1982). There was also proposed a process for producing a C.sub.8 -aromatic hydrocarbon mixture having a relatively low content of non-aromatic hydrocarbons, which comprises distilling a naphtha reformed oil, polymerizing olefins therein which are difficult to remove by distillation treatment alone and become a poison on the xylene isomerization reaction catalyst, and again distilling the residue (Japanese Laid-Open Patent Publication No. 181036/1985). The amount of non-aromatic hydrocarbons in the resulting C.sub.8 -aromatic hydrocarbon mixture so obtained is usually 0.05 to 3% by weight, typically 0.1 to 2% by weight. The non-aromatic hydrocarbon usually consist of 70 to 80% by weight of C.sub.8 -C paraffins and 20 to 30% by weight of C.sub.8 -C.sub.10 naphthenes.
If the C.sub.8 -aromatic hydrocarbon mixture produced as above without going through the solvent extraction step can be directly used as a material in the xylene isomerization reaction, the cost of the material can be reduced, and the p-xylene manufacturers can offer p-xylene at a decreased price.
When the C.sub.8 -aromatic hydrocarbon mixture containing a small amount of non-aromatic hydrocarbons is continuously used as a starting material for p-xylene production by isomerization of xylenes, the non-aromatic hydrocrbons gradually build up in the process because of the reduced ability of the catalyst to convert the non-aromatic hydrocarbons, and adversely affect the xylene isomerization reaction itself. For example, if the reaction conditions are rendered severe to elevate the above converging ability, the yield of xylene will be decreased and finally, it becomes necessary to purge the the accumulated non-aromatic hydrocarbons out of the process.
Thus, although various processes have been proposed to date for the production of a C.sub.8 -aromatic hydrocarbon mixture having a relatively low content of non-aromatic hydrocarbons, an industrial process for producing p-xylene using such a C.sub.8 -aromatic hydrocarbon mixture as a material has not yet been perfected.
Recently, some literature references disclosing the intention of using the above raw material have been published. Examples are U.S. Pats. Nos. 4163028, 4312790, 4385195 and 4224141 and European Patent No. 102716 of Mobile Oil Corporation, U.S.A. Since in any of the methods disclosed in these references, the C.sub.8 -aromatic hydrocarbon mixture containing non-aromatic hydrocarbons is fed under very severe conditions (for example, at 427.degree. C. and 230 psig) onto zeolite whose acid activity is reduced, side-reactions (such as disproportion and ring-cleavage reaction) consequently occur, and a decrease in the yield of xylene is noted. Furthermore in the continuous feeding of the hydrocarbon stock under severe conditions, the activity of the catalyst decreases with time, and the activity of converting the non-aromatic hydrocarbons is also reduced. As a result, these components build up in the recycling system, and are likely to reduce the efficiency of p-xylene production.